Around here, in small town WI, Subway is often the only fast food restaurant (maybe restaurant of any type) in a town, frequently connected to a gas station. While one might think that being the only game in town they're set up to do good business, in reality it seems to mean that there really isn't enough business where they are located to be able to PROPERLY run a restaurant of any type. So you often get a bad experience. Underpaid staff who don't care. Understaffed period. Not enough turnover to keep product fresh. The store barely makes money so they don't spend an extra penny where they can help it. These are the kind of stores where you are likely to show up at lunch time on any random day and find a hand-scrawled note taped to the door by some manager (who isn't qualified to manage anything) saying "Sorry we're closed, no one wants to work anymore." Which might be true, but a manager and franchisee of a location needs to do whatever it takes to not let that affect the restaurant's operations, even if that means working 15 hours a day themselves until they figure something out. Other times they might be open but you could literally be waiting a half hour for your food if there are a few people ahead of you. One thing about Subway, the way they work one person can take your order, make your food, and be the cashier. And because that CAN happen it often WILL happen. Even during busy times, there might be a single employee who doesn't really care and doesn't know how to be efficient. At least at something like McDonald's, at a minimum there typically needs to be someone at the counter and someone in back making food.
I think Subway also attracts a type of franchisee who really doesn't care that much as long as they make some money and will oversaturate their market with multiple substandard stores vs one really well-run one if they think that will be more profitable. And Subway itself apparently doesn't have a problem with that. In one small city I used to live, there were 3 Subways within a couple miles (and a fourth another couple miles across the river in a different city, but same population center). All operated by the same franchisee. Was back there this weekend and noticed that only one of the 3 is still open.
Fast food franchises in general have long been set up with such onerous buy-in and liquidity requirements that for an individual to own one you really have to have made your money somewhere else. Often this has been some sort of middle-aged business owner (contractor, car dealer, store owner, real estate investor, etc.) who needs someplace to park their money. The pitch to the franchisee isn't "start a business you're passionate about and make your neighbors happy by serving them great food" it's "buy in and with our name you're almost guaranteed to get XX% ROI." So franchise owners often don't really care.
Chick-fil-a is a notable example of a different approach. As mentioned, their franchises have remarkably low requirements for a business that, at least to this point, is almost a guarantee to be successful. But they also expect you to run that restaurant like it's your own baby and protect the CFA brand. You aren't allowed to have other non-passive businesses and are expected to be at the restaurant actively managing on a day-to-day basis. You do need to show that you have the business skill and do need to come up with $10k on your own (not borrowed). But other than that, they are more looking for people who are passionate about running a restaurant, and a Chick-fil-a restaurant in particular, than an experienced hard-nosed businessman who is great at squeezing every possible dime out of a business. That has worked unbelievably well for them, though I do have concerns that with expansion, standards will start to slip.